1. Field of the Invention
The invention in general relates to reducing the thickness of certain semiconductor structures.
2. Description of Related Art
In the field of semiconductor device fabrication it is often desired to remove a semiconductor layer from an adjacent layer of other material. For example, the semiconductor layer may be of silicon and the adjacent layer may be of an oxide of silicon, such as silicon dioxide. This type of arrangement is found in an SOI (silicon on insulator) structure which includes a base layer of silicon adjacent to an intermediate layer of silicon dioxide and a top layer of silicon.
Several methods are used or have been proposed for the removal of the base silicon layer. These include plasma etching, grinding, lapping and wet chemical etching. Plasma etching has been demonstrated with fluorine and chlorine based etchants. These etchants however, have a relatively low selectivity to the oxide, for example 100:1. Otherwise stated, these etchants will etch through the silicon at a rate X, and through the silicon dioxide at a rate Y, where X/Y is 100. This makes it difficult to stop the etching process when the oxide layer is reached. Grinding and lapping leaves an objectionable and unacceptable rough surface leading to uneven subsequent etching.
Wet etches include the use of potassium hydroxide, tetramethyl ammonium hydroxide and hydrofluoric/nitric acids. Some of these etchants have a better selectivity to the silicon dioxide, however they attack the edges of the semiconductor structure, even if a protective coating is applied. This leads to objectionable rough edges and microcracks in the structure.
It is a primary object of the present invention to obviate the drawbacks of the conventional thinning processes currently used.